1. Field of the Inventions
The inventions generally relates to video decoders, and more specifically to input video signal shaping, and even more specifically to providing constant video signal levels.
2. Description of Related Art
There is a large surge in the use of digital video devices today. Examples include: digital televisions, LCD (Liquid Crystal Display) TVs and monitors, DVD (Digital Versatile Disc) recorders, personal video recorders, PC (Personal Computers) video cards, video capture and streaming applications, and video conferencing. In many cases, these units need to receive an analog video signal, which may be one of the composite signals, such as NTSC (National Television Standards Committee), PAL (Phase Alternating Line) or SECAM (Sequential Couleur Avec Mémoire); s-video; component video or RGB (Red, Green, Blue). It is then desirable to produce the proper digital output, such as eight or ten bit ITU-R BT 656. It is preferred that all the video decoding be done in a single chip for all of these formats. The decoder not only has to handle composite signals, which means it must be able to determine the chroma and luma values, but it also must handle vertical blanking interval (VBI) data and handle VCR signals, which may be unstable signals.
In many cases, the actual signal level being received will droop over time due to various leakage currents. This droop will result in the signal effectively getting darker, i.e., having less amplitude or less luminance. It is very desirable, of course, that the picture remain at a constant brightness level as compared to the transmission level, and therefore, it would be desirable if some way to address the drooping was developed.
Because the composite video signal is usually AC coupled into the decoder, a DC bias level of the internal signal must be developed. If this DC bias level is not correct, all brightness values will be incorrect. It is very desirable, of course, that the picture remain at the proper brightness level as compared to the transmission level, and therefore, it would be desirable to provide the proper DC bias level.